Barrels: 
All variations and serial number ranges of the military 1911/1911A1 pistol used specifically marked and identifiable parts.  Often these parts can be identified by machining marks, finishes, or specific unique characteristics, such as a bevel or rounded edge even if the normal mark is missing.  Collectors attach great importance to a pistol having all of its “correct” if not “Original” parts.  However many of the small parts such as springs, pins and disconnectors have no specific markings and are not readily identifiable.  Many, are identifiable as “GI” in contrast to “Aftrer market”.  However distinguishing between the  latter two categories is sometimes difficult and fuzzy.  None the less, collectors attach great importance to a pistol having all of its “correct” if not “Original” parts. Reference Charles Clawsons “Colt .45 Service pistols”.

Colts from serial #700001 to serial #710000 (1924 production) will have barrels with a Gothic “H” and a Gothic “P” (No serifs) that share a common line, and are thus a single mark, and is sometimes called the “Interlaced HP”, on the area visible through the ejection port. This marking will normally be correctly oriented when the pistol is viewed pointing to the Right and the barrel is viewed from above the pistol.  These pistols sometimes known as “The Transition” or “1924” will generally have a partially struck very large “K” in front of the lug. (SEE FIGURE 10 BELOW)

Colts from serial #710001 to serial #712349 (1937 production)  will have barrels with a Gothic  “COLT .45 AUTO” (SEE FIGURE 6 BELOW) marking on the chamber area, along the axis of the barrel, at about 7:30 O’clock, when the barrel is viewed from the rear, with the lug down. A “P” mark will be applied to the left lug.  These barrels are blued with a semi-matte semi-reflective finish.  Clawson mentions “Top of barrel chamber bright unfinished, the same as commercial barrels.  Full bluing in the military style began at or near the end of 1937 production.”  At this point, insufficient pistols have been examined to determine the point when full bluing occurred and the chamber area was no longer in the white.  However it is likely that barrels started in the white. The "G" stamp started to appear on the bottom of the barrel forward of the lugs at serial 711,000 and represented it is a "Government" contract barrel this stamp was was present until around May 1943. At about s/n 935,000, the G mark was eliminated due to the suspension of commercial sales on January 23, 1942 by order of the United States Government thus removing the requirement to differentiate if the barrel was for government contract or commercial sales. The are three different G variant barrels; A large G, a small G, and a large G in combination (over the top of) with either an N, F or 7. Those marked with the N, F or 7 were the last ones before the G was omitted.
 

Colts from serial #712350 to somewhere in the 2 million range number range, (1945 production,) will have barrels with a Gothic  “COLT .45 AUTO” marking on the chamber area, along the axis of the barrel, at about 7:30 O’clock, when the barrel is viewed from the rear, with the lug down. A “P” mark will be applied to the left lug.  These barrels are fully blued with a semi-matte semi-reflective finish. Forward of the lugs on the bottom of the barrel the "F" or "7" stamping has been observed starting at Colts produced around serial number 930,000 until the end of production. (SEE FIGURE 12 BELOW) .

"S" stamping forward of the lugs 711,000 to 935,000 (approximately): Most or all of the Commercial to Military conversion pistols have the "S" marked barrel as they were originally Commercial pistols and would have had "S" for "Sales" marked barrels. Colts apparently used most of its remaining commercial parts in pistols following the C to M conversions, as Commercial slides (With commercial markings) can be seen in pistols up through the Canadian group.  The S marked Colt barrels will sometimes exhibit an almost fire blue color on the chamber area. Apparently these barrels had already had the chamber area polished out to the white, and then were heat blued to meet military requirements.

Colts from somewhere in the 2 million range number range, (1945 production) will have barrels with a Gothic  “C” in a square on the Left lug.  A “P” mark will also usually be applied to the left lug.  These barrels are fully blued with a semi-matte semi-reflective finish.

Postwar replacement GI barrels have been observed with “SW”, “H&R”, and “TZ”.

Early Commercial marked barrels from 1916-1918 have also been observed on M1911s of that era . See Figure 2 below

Colt .455 barrels:
These pistols are commercial production and not USGI, however some were used by British military.  Colt barrels in .455 calibre have a E for English or W for Webley stamp forward of the lugs on the barrel for identification.  These were normally found only in commercial pistols with slides marked “Calibre .455”.  These pistols had a slightly larger magazine well that would accept the special .455 magazines required for proper feeding of the .455 cartridges.  These magazines will normally not fit into a .45 1911 pistol.  A .455 barrel installed in a 1911 would possibly function, but would not headspace properly and create a dangerous situation. A .45 barrel installed in a .455 pistol would probably function so long as .45 cartridges were used.

Ithaca:
Used mostly High Standard barrels with "HS" on one lug and a "P" on the other lug (See figure 4). Barrel use for Ithaca was much like barrel use at Remington Rand. Some Colt and Springfield (See figure 8B) barrels were in early production and Flannery Bolt Co. barrels seen in 1944-45 See figure 9 below, Flannery barrels have an “F” stamped on the upper part of the Right lug.

Remington-UMC:
Most common is a “P” located in the area of the barrel visible through the ejection port. This “P” marked barrel should not be confused with early commercial barrels with a “P” at the front of the ejection port area that was in the white.  Also found is P located on right side of lug. See Figure 5 below

Remington Rand:
Used mostly High Standard barrels with "HS" on the upper Right side of the lug and a "P" on the Left side of the lug. Very early Remington Rands (with “NEW YORK” marked slides and Type II) normally have “Colt .45 AUTO” barrels.  Some Flannery barrels were observed from 1944-45 (See figure 9), these barrels have a P on the Left side of the lug and a F on the upper Right side (1.7-8 million serial number range). See figure 4 below for image of HS barrel.

Singer:
Used barrels made by Singer that were marked only with a “P” on the Left lug, and were finished with a fine but semi-reflective blue finish.

Springfield:
Springfield barrels always had an "S" marked on either the left or right side of the lug. Early barrels were marked “S” on right side of lug (See figure 8A). After about 100,000 there is also a "P" marked on the left side of the lug.  An “A” ,an  "M", or an "MD" will usually be found on the rear of the barrel hood on on most early pistols as well. Later barrels had the "S" and The "P" located together on the left side of the lug. (See figure 8B)

US&S:
Used a High Standard barrel. The upper Right lug has an "HS" mark and the left lug has a "P" mark. See figure 4 below

Note: Flannery Bolt Co. barrels were first used in 1943 for field service but some were allotted to Remington Rand and Ithaca in 1944 and 1945 for their production needs. (See figure 9)

Note 2: Parkerized/blued bullet feed ramp finish:
Bullet feed ramps were blued in receivers to serial number 710,000, then at SN 710,001 they were machined after the receiver was blued or parkerized, leaving this area of the gun in the white and unfinished. This area should show machine marks and not be polished. In mid-1944 Colts bullet feed ramps were parkerized, this began at approximately SN 1700000.

Example Barrels (Click on image for larger version)
Figure 1A WWI Colt Military (About S/N 120000 to 400000)	Figure 1B WWI Colt Military (About S/N 400000 to 629500) "Interlaced HP"	Figure 2 Early Colt Commercial	Figure 3 WWII Colt Military
Figure 4 Remington Rand/Ithaca/US&S  High Standard barrel	Figure 4b Remington Rand/Ithaca/US&S  High Standard barrel other side	Figure 5 WWI Remington UMC	Figure 6 Colt WWII
Figure 7a Colt WWI Production	Figure 7b Colt WWI Production	7A&7B Shows the gothic H in a vertical position on the rear of the barrel extension/hood observed on pistols from approximately S/N 6500 to 25000.
Figure 8A WWI Springfield barrel	Figure 8B Ithaca Springfield barrel also found on WWII arsenal rebuilds.	Figure 9 Flannery Barrel 1943-45 right and left lug (F and P)	Figure 10 1924 Colt Transition barrel
Figure 11 Colt WWII G marked barrel Approx. 711000 to 935000	Figure 12 Colt WWII F marked barrel Approx. 930000 to end
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